& Run the previous command in the background ls & && Logical AND if [ “$foo” -ge “0” ] && [ “$foo” -le “9”] || Logical OR if [ “$foo” -lt “0” ] || [ “$foo” -gt “9” ] ^ Start of line grep “^foo” $ End of line grep “foo$” = String equality (cf. -eq) if [ “$foo” = “bar” ] ! Logical NOT if [ “$foo” != “bar” ] $$ PID of current shell echo “my PID = $$” $! PID of last background command ls & echo “PID of ls = $!” $? exit status of last command ls ; echo “ls returned code $?” $0 Name of current command (as called) echo “I am $0” $1 Name of current command's first parameter echo “My first argument is $1” $9 Name of current command's ninth parameter echo “My ninth argument is $9” $@ All of current command's parameters (preserving whitespace and quoting) echo “My arguments are $@” $* All of current command's parameters (not preserving whitespace and quoting) echo “My arguments are $*” -eq Numeric Equality if [ “$foo” -eq “9” ] -ne Numeric Inquality if [ “$foo” -ne “9” ] -lt Less Than if [ “$foo” -lt “9” ] -le Less Than or Equal if [ “$foo” -le “9” ] -gt Greater Than if [ “$foo” -gt “9” ] -ge Greater Than or Equal if [ “$foo” -ge “9” ] -z String is zero length if [ -z “$foo” ] -n String is not zero length if [ -n “$foo” ] -nt Newer Than if [ “$file1” -nt “$file2” ] -d Is a Directory if [ -d /bin ] -f Is a File if [ -f /bin/ls ] -r Is a readable file if [ -r /bin/ls ] -w Is a writable file if [ -w /bin/ls ] -x Is an executable file if [ -x /bin/ls ] ( ... ) Function definition function myfunc() { echo hello }